Casting from interface type

Please consider this small code :
--8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<---
procedure Proc is
type I is interface;
type T is tagged null record;
type DT is new T and I with null record;

var_t: T;
var_tc: T'Class := var_t;

procedure Conv(p: in I'Class) is
begin
if p in T'Class
then
var_t := T(p);
var_tc := T'Class(p);
end if;
end Conv;

var_dt: DT;

begin

Conv(var_dt);

end Proc;
--8<-----8<-----8<-----8<-----8<-----8<-----8<-----8<---

Using the latest GNAT 2006, I receive the following errors:
proc.adb:14:19: invalid tagged conversion, not compatible with
type "I'Class" defined at line 3
proc.adb:15:21: invalid tagged conversion, not compatible with
type "I'Class" defined at line 3